When I think of signature phrases, or catch-phrases, I typically tend to think of superheroes (I imagine a lot of you do as well). Spider-man is known for the piquant phrases he uses to taunt villains, and he's not alone. Ordinary novel or story characters certainly don't have to be so flamboyant, but I find that signature phrases can actually help them as well.
A signature phrase doesn't need to be so incredibly sassy that it's unmistakable. Though that typically works well for superheroes, it's usually too much for a novel character - even an important one. On the other hand, you can give relatively innocuous phrases to characters as well.
Here's an example. One of the characters in my novel, a man named Erex, tends to say, "In fact." Not once or twice, but in just about every lengthy speech turn that he gets. It fits with his character, because he's a sort of guidance counselor, and the phrase helps him to sound the role. It's only two words, but it gives him a very distinctive speech quirk to match his personality. Here's one of his interactions:
"Are you feeling all right, sir?" Tagaret asked, shaking the hand he offered.
Erex nodded. "I am well. You're kind to ask. In fact, I needed an excuse to come and find you."
"I didn't forget to report my grades, did I?
Erex smiled. "No."
"Then Father invited you."
Erex's smile diminished. "In fact, no. However, when I became aware that Garr had invited certain individuals, I made sure to come. He knows I'm here."
"Individuals?" Oh, gods, how just like Father – he'd probably invited half the cabinet!
Erex spread his hands. "Individuals with whom I'm quite well acquainted, in fact, so I thought I might be able to help."
One of the reasons that signature phrases can work so well is that people have speech habits, and those will stand out. I'm sure you know someone who uses the phrases "you know" or "like." Do you also know someone who overuses one of them, or both? I knew a young woman once who said "you know" so often that it nearly interfered with comprehension. She knew how to say "you know" compressed into an inhale!
I was revising through my manuscript yesterday and one of my characters said "Oh, dear." This surprised me. On the other hand, it was rather a nice thing, because my main character typically says "oh, no" when something bad happens, and I was happy to hear the other character saying something different. Then a little bit later I ran into someone else saying "Oh dear" and became alarmed. Maybe it was a phrase that happened to be on my mind that day, and suddenly it started sneaking into everybody's dialogue. That didn't make me happy. However, I decided I'd change the second instance of it to "Dear me" instead. I don't think that "dear me" will become anything like Erex's "in fact" for the character involved, but it was useful to think through how two different characters would be distinct in their reaction style.
In fact, ;) this is what we're really looking at - ways to keep characters distinct from one another. I would hope that we also spend a lot of time making characters distinct in other ways - personal history, emotional range, backstory, cultural background, etc. But it's also good to take a look at a character's first reaction to a shock, and make sure that he or she doesn't have the same reaction every time - and certainly that he or she doesn't have precisely the same reaction style as another character. In this respect, signature phrases can become very helpful.
It's something to think about.