Peter Sellers performance here is one very much about character creation and character persistence. Chance is not a character who changes at all since he is in fact a simple mentally retarded man. Sellers' creation of Chance is perfectly done by him. Everything aspect of Chance is finely attuned, to make Chance an endearing character. His whole manner in his very particular manner of walking and moving, and that simple Stan Laurel voice perfectly amplify the emptiness of which the character is.
Peter Sellers makes every motion and movement of character completely fit with that character, from his very gentle way of speaking, and moving, to his way of always turning toward the television. Sellers shows that is simply where Chance sees everything and knows everything therefore he turns to the television for any sort of idea or thought. He imitates what he sees in a slow, completely childlike way, in which he just follows what he seems to see it as right.
Sellers does something very important as Chance and that is because he stays completely consistent throughout the performance. He never tries to show an actual change in Chance which is important, he shows that Chance will not change in his personality for this is the way Chance is and always will be. If Chance changed at all it would be incorrect for the character, and Sellers always keeps with the character, and keeps with the proper way of the character throughout the film.
Sellers makes Chance into a fully realized and believable as a character, he makes Chance somehow real despite the strangeness of Chance. He also manages to add a certain amount of heart to Chance despite Chance never showing any overt emotion. He somehow makes Chance charming, without Chance trying to be due to his simple but incredibly effective characterization of Chance. Sellers even more amazingly actually makes the plot believable for about half the movie, because of this unique characterization. The second half not being believable is not his fault, the story just simply takes it further than it really can go.
Chance does not change really except in a very small way which is earned by Sellers, since he showed Chance slowly learn this change from others and television, which is his emotional recognition of death. Sellers slowly shows this very much earned change, which is with the character because he shows a childlike recognition of death at the end of the film which Sellers makes very effective, and consistent with Chance.
Now Sellers lacks a character ark and Chance really only has one very minor change throughout the performance, and one could easily say this is not really an extremely challenging character, since he does not change. Chance is a simple character but he could have easily gone completely wrong with another actor. Sellers though finds the right tone for his performance, and brilliantly creates Chance into an endearing and effective character.
