Steve Martin plays the abusive boyfriend of Seymour's love interest Audrey (Ellen Greene), that's not the important part of his character. The important part is that being a sadistic dentist which is portrayed in a hilarious song number as he describes his reasons for his profession which is simply because of his love of causing things pain. Martin is usually best known for his dead pan comedic performances but this one is a broadly comic portrayal which was probably unavoidable considering it involves a song sequence. Anyway Martin plays the role as basically a twisted Elvis impersonator as he tries to inflict as much pain as possible to other around which is best shown in his first and best scene his song.
Martin's performance is completely hilarious in the song as he delivers with the song particularly brilliantly. He changes from one moment to the next with whether it is emphasizing the impersonation, or at times kind of the big singing known in the fancier type of romantic musicals, and other times with a psychotically intense voice. Each switch is perfectly place to accentuate the hilarity of the sequence. To only add on top of that his physical performance is also comedic gold as he wields his arms almost as weapons of death constantly threatening all humans who may be nearby him. Martin pulls out every single possible ounce of humor in the scene and he successfully makes the whole dentist song scene one of the strongest in the film.
Martin other great scene is when he has a patient played by Bill Murray who unfortunately for him is a masochist. The two comedians play off each perfectly with with Martin going as intense as possible in his the pleasure he gets out of his vicious attempts to cause the man discomfort, and Murray playing off it by showing the man to love every single moment of it to well an orgasmic extent. The best moment of the scene though easily comes when Martin's character throws him out and it is ridiculously funny just how much disgust Martin displays in the dentist, as it is quite obvious that the dentist only likes it when he is the one having fun. That also is a great scene, and Martin is only in about four scenes to begin with.
Well the other two are a little less funny, but then again that is a bit hard to match considering the quality of the other two. Don't get me wrong he is still quite enjoyable in these two other scenes particularly his expression when he realizes he's going to die from his laughing gas machine. They are not nearly as entertaining as the other two but that is far less Martin and much more caused by the limitations presented to him. Nevertheless Martin proves himself quite capable in the broader style of comedy and is still one of the best parts of the film. Although he's merely good in two of his scenes opposed to being great like he is in his other two scenes, but I suppose that's good enough.