For the character's reaction to the stimuli work, the writer needs to know the characters. Think of it this way, a bully doesn't become kind and helpful, unless there's a reason. Now a character who has been bullied by one other of the characters had better suspect something is afoot when the bully acts out of character. His reaction to this stimuli from the bully would have a subtle difference from a time when the bully is acting in character.
Knowing your characters makes this stimuli reaction sequence an important tool for the writer. Any reaction must mean something. If it doesn't there's no reason for that to be the reaction. As a writer, there are times when I fail to make this stimuli-reaction-action sequence work. This means a lot of rewriting.
The reaction must be timely. To have a reaction come hours or days later may be what happens in real life. There are those times when we think I should have done or said this or that. The characters in your story can't do this. They need to react and act with immediacy.
Their action or reaction needs to be in character and be part of their motive for the scene and the story. This can be tricky but it done right the reader is pulled along. This is because the stimuli=reaction-action sequence works out in a reasonable way. Once this is learned, the writing becomes easier.