Bob would like to help Alice, but just can't.
The moment where as much as they like to do it, they just don't.
A subtrope of the Rule of Drama that says, "Anyone on whom a character is counting for help won't simply give aid. If the request is not flatly refused outright, the other character will express reservations or cite prior obligations as an excuse for not helping out." The contrast with Real Life is largely a question of degree. An initial refusal is more likely or more prevalent in fiction because of the need to lengthen the story and/or put up obstacles on top of the myriad other reasons someone may have to refuse a request. If you're a betting sort, don't bet that a request made in a work of fiction will be met with an immediate "Yes." or "Sure!" The P.I. knows a fence who owes him a favour? The fence denies the relevant knowledge or is afraid of what someone else will do to him if he talks. A guy asks his best friend to help him move? The friend has his in-laws visiting and can't help. A kid wants an older sibling to help with a tough homework assignment? Sorry, the sibling has team practice or a hot date. The reluctance (or even outright refusal) to help provides a new obstacle for the protagonist. After all, we can't solve problems too quickly, or else there's no story. After the prospective helper invokes this rule, a number of things can happen. If the asker has any leverage they can use, it will be mentioned next (prior favours the asker has performed, embarrassing information the asker knows, and suchlike). If the reason(s) for the reluctance seem valid, the asker might withdraw the request. If the refusal is or seems final, the asker will try someone or something else. Refusal of a villain's "request" can lead to assault or murder. Sometimes (particularly in comedies), the reluctance is a prelude to negotiations that result in the asker being committed to doing something in return. Often, the return favour will involve some trouble and/or expense; in comedies, this usually entails potential for public embarrassment or closer cooperation with someone else who is trying, obnoxious and/or unpopular. If this rule is invoked multiple times, the result could be a Chain of Deals. The more useful the help is likely to be, the more likely this reluctance will surface. A powerful character invoking this rule can lead to a Deus Exit Machina, most often in the form of some prior or overriding commitment as the reason for both their refusal to help and their exit. Because the Rule of Drama and the Anthropic Principle underlie many of the excuses for Holding Back the Phlebotinum, any character who has access to or control of said phlebotinum is almost guaranteed to follow this rule. It's also behind the reluctance of any usually helpful and well-connected friends (be they friends with underworld connections or those with more legitimate authority and resources) to be less so on a particular occasion.
Proposed Indexes on Launch: Rule of Index, Bargain Tropes, Comedy Tropes, No Examples, Please, Omnipresent Tropes, Universal Tropes,