One thing I've learnt (and had to relearn the hard way several times over in the last decade) is that in sitcoms, it is very hard to make guests characters as funny as the regular characters. This is easy to forget because its always counter-intuitive. New guest characters bring fresh ideas, different situations and new angles, so surely they're easy to make funny, right? Wrong. Now why is this?
Let's take a step back and remember what a sitcom is. It's not a comedy about new situations and funny lines. It's a half-hour story about characters that we know and love in their usual surroundings. The audience are used to seeing the world through their eyes. The audience love the regular characters. They are invested in them. They are not going to love a strange shop assistant or over-the-top hairdresser in the same way. They know the regulars and what their hopes and dreams are and therefore why their lines are funny. One-off characters bring uncertainty, which is why the most successful, funniest and most useful ones tend to be very quick and easy to understand and larger than life.
As Usual, Seinfeld Leads The Way
I think this is why the writers of Seinfeld were so successful at introducing and occasionally reprising their guest characters. The successful and memorable ones tended to be completely extraordinary and at least partly based on truth eg. The Soup Nazi or The Bubble Boy. Their names explained exactly who they were so the audience were up to speed straight and we could get on with the jokes. A catchphrase helped that along too ('No soup for you!'') But what we enjoyed the most about these characters is the reactions of the regular cast to them. Seeing George humbly bowing to the Soup Nazi and then being cheated of a bread roll and then being banned is funny - because it's George. And then there was the unpleasant incident with the Bubble Boy... Other characters in Seinfeld weren't funny at all but downright annoying, intentionally so because they put the regular characters into awkward or unpleasant situations, one of the most obvious examples being the infuriating comedy hack Banya. Banya made Jerry funny.
So, it's worth looking at your one-scene character - and remembering that the audience are not invested in this character as much as you by a long way. If possible, give the jokes to the regular character. Quite often, a joke can be switched around so our hero is able to have the punchline and it still feels like it's all in character. As writers who are maybe slightly jaded about our regulars, we often get excited about the new guy, the new voice, the new attitude and therefore the new jokes that are possible. The audience are a lot less interested in this guy than you are. Wherever possible, give your jokes to the regulars.