SME (pronounced SMEE) is your subject matter expert.  SMEs need you to document their work, and you need them to review drafts and to provide feedback.

Provide your SMEs with a draft that's as close to final as you can make it. Many reviewers are busy, and they don't see documentation review as a high priority. (Not everyone likes to read. It's a chore for many.)

Find a way to flag your document with highlights or comments. You'll get answers faster if you make it easier for your reviewers. (Also if they are reviewing something for the second or third time, be kind and let them know what has changed.)

I use email to get answers to questions. I try to collect them as I work. Then I organize them and send them in an email. (Don't overburden your SME with 100 questions.) I prefer email because of the electronic trail. But check with your SME to see what his/her preferred method of communication is.

I know technical writers who prefer IM-ing for quick hits. It's a great tool for that, but be aware that too many are seen as an interruption.