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Smokin' Hot~
From the previously mentioned interview with Sakurai in Nintendo Dream where he discussed how Bowser Jr. was almost cut from Smash, Sakurai also took the time to talk about some of the characters that did make the cut, specifically Mii Fighters, Dr. Mario, and Duck Hunt.
---Mii-Fighter
Sakurai: This time customization was a factor, I thought that we had to take a step towards being able to create something to your own liking during development. Although, we were thinking a lot about whether people would be happy to see the Mii Fighter as their opponent. For example if "My mommy" would come up as an opponent, would that really become a heated battle...? As we thought about that, if the character were to look and come out nonchalantly like in Mii Plaza, this would have been a definite failure. In the end by having the head of the Mii and changing up the body as well as giving them proper moves we managed to bring it into existence.
Dr. Mario
Sakurai: Actually all "Model-change"-characters that we have now were originally just supposed to be color-configurations. Just like the Little Mac-wireframe or how Olimar and Alph are included, we planned to just make the look and maybe the voice a possible configuration. But, when talking about Dr. Mario, including his abilities he had a lot of individuality in Melee compared to Mario. Also there were fans that said things like "I'm a doctor!", or "A back-alley doctor!" when they chose the black color-config. When you have such fans and you just change the character-model and leave it at that and it becomes "The same as Mario", we thought people might get angry (laughs). That's why we forced ourselves and changed that. But, unlike we did in the past when we separately added abilities and monitored the results, this time we used the customization-system and entered the adjustment-process by starting with a "Mario that's a bit heavier, with added attack-power". Although even then it still took a lot of work. During the regular adjustment-process we have each fighter battle all others and try to grasp the "strong" and "weak" problem areas, but in this case we had Dr. Mario go against Mario and tried to adjust it so it would most likely result in a tie. We did the same for Marth and Lucina, as well as Pit and Black Pit.
Duck Hunt
Sakurai: You think Duck Hunt is a rather minor character, right?
Nintendo Dream: We...think so. (laughs)
Everybody: (laughs)
Sakurai: Please try to have a look at the sales-numbers. It might be the most-sold shooting-game in the world. It might be because it came bundled with the NES outside of Japan, if you look at worldwide numbers you come up with a huge number. I thought to create a character based on that would be appropriate, so we included that character from the very beginning of the project.
Nintendo Dream: So it was a candidate all along for the whole Smash Bros.-franchise?
Sakurai: No, that would be exaggerating...in past titles we had our hands full with the characters we included back then. Even if we had tried even harder back then we couldn't have included it. But this time it was fun to do "Duck Hunt" and its shooting-like presentation. Like having a third person intervening the action, being able to say "It's actually a trio" was fun. That's how we finalized it during development of the concept, saying "That's how we can make it work!".
SmashCapps loves all of the characters in Smash and is glad they were all included. Maybe you can tell him your favorite character after following him on twitter.
Source: Neogaf

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Sakurai: This time customization was a factor, I thought that we had to take a step towards being able to create something to your own liking during development. Although, we were thinking a lot about whether people would be happy to see the Mii Fighter as their opponent. For example if "My mommy" would come up as an opponent, would that really become a heated battle...? As we thought about that, if the character were to look and come out nonchalantly like in Mii Plaza, this would have been a definite failure. In the end by having the head of the Mii and changing up the body as well as giving them proper moves we managed to bring it into existence.
Dr. Mario
Sakurai: Actually all "Model-change"-characters that we have now were originally just supposed to be color-configurations. Just like the Little Mac-wireframe or how Olimar and Alph are included, we planned to just make the look and maybe the voice a possible configuration. But, when talking about Dr. Mario, including his abilities he had a lot of individuality in Melee compared to Mario. Also there were fans that said things like "I'm a doctor!", or "A back-alley doctor!" when they chose the black color-config. When you have such fans and you just change the character-model and leave it at that and it becomes "The same as Mario", we thought people might get angry (laughs). That's why we forced ourselves and changed that. But, unlike we did in the past when we separately added abilities and monitored the results, this time we used the customization-system and entered the adjustment-process by starting with a "Mario that's a bit heavier, with added attack-power". Although even then it still took a lot of work. During the regular adjustment-process we have each fighter battle all others and try to grasp the "strong" and "weak" problem areas, but in this case we had Dr. Mario go against Mario and tried to adjust it so it would most likely result in a tie. We did the same for Marth and Lucina, as well as Pit and Black Pit.
Duck Hunt
Sakurai: You think Duck Hunt is a rather minor character, right?
Nintendo Dream: We...think so. (laughs)
Everybody: (laughs)
Sakurai: Please try to have a look at the sales-numbers. It might be the most-sold shooting-game in the world. It might be because it came bundled with the NES outside of Japan, if you look at worldwide numbers you come up with a huge number. I thought to create a character based on that would be appropriate, so we included that character from the very beginning of the project.
Nintendo Dream: So it was a candidate all along for the whole Smash Bros.-franchise?
Sakurai: No, that would be exaggerating...in past titles we had our hands full with the characters we included back then. Even if we had tried even harder back then we couldn't have included it. But this time it was fun to do "Duck Hunt" and its shooting-like presentation. Like having a third person intervening the action, being able to say "It's actually a trio" was fun. That's how we finalized it during development of the concept, saying "That's how we can make it work!".
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It's always interesting to see the thought process of Sakurai and this is a cool incite as to why certain characters were chosen to be a part of Smash as well as how they went about making them so unique.
SmashCapps loves all of the characters in Smash and is glad they were all included. Maybe you can tell him your favorite character after following him on twitter.
Source: Neogaf