Learn English – What’s the best way (word or phrase) to say “out of pattern”

A schtick refers to a comedian's style and choice of topics in general, and could include facial expressions or actions as well as words or style of delivery. It doesn't refer to a specific gag that is repeated often (although a repeated gag might be part of a comedian's schtick). It also often distinguishes a comedian's on-stage role from their actual personality:

Gracie Allen typically played a dingbat character, but that was just her schtick. In real life she was an exceptionally intelligent person.

A running gag is a joke that is repeated in a given work (book, film, tv show, etc.) or series of works. It's not necessarily spoken by the same actor at each occurrence.

"gag routine"(*) is not a commonly used idiom in English.

A signature gag would be immediately understood to have the meaning you suggest, but it isn't an idiomatic phrase, it's just two words used together that mean what you want. However, the phrase is "signature line" is probably more common than "signature gag". For example,

"Take my wife, ... please" is Henny Youngman's signature gag [or signature line].

A catchphrase is another related term, meaning a particular phrase used by a performer (not necessarily a comedian) throughout their career. This could include Henny Youngman's "Take my wife," or John Wayne's calling people "pilgrim". (Thanks @JoeSteele)

If by early, you mean that you want more time from him to make a decision, (that's what the situation tells me), I'd recommend a legitimate use of

Sorry, but as you see choosing the [an adj that describes the quality of the way you choose the best; e.g.: optimum, most productive etc.] way is hard to do instantly/at the moment. I'll need more time to put my thoughts together and will be able to respond in ...