What's the difference between "plump" and "plump up"?

Question

What's the difference between the two?

Example sentence:

Mary got plastic surgery to plump (up) her lips.

Answer 1

Plump can be a verb "to plump up" or an adjective "Mary's lips are plump".

In your example, the "up" should be there because this is verb usage, but might be omitted in casual speech.