Put words in his/her mouth! You work hard to be competitive, don't skimp here if you have the opportunity. In your career, it's quite likely you'll be drafting your own letters on behalf of a writer, so get used to being your own advocate.
For one letter writer, I put together a short 1 paragraph motivations statement. I also listed my biggest activities/accomplishments (like research, volunteering, etc.) and put like a 1-2 sentence blurb about what I did and what I learned from it. Even bolded key adjectives like leadership and initiative. Also included a basic CV. If they wanted to write it off the top of their head, they could. If they wanted to use my CV, they could. If the wanted to use some of the text I put together, they could. If they're writing you a letter, they want it to be an effective one, so help out (IMO).