The book in question is about time travel Stewart is a translator, knowledgeable in French and Latin, and Leonid is a medical doctor. I do not know how to flesh these out, but it seems necessary to add them
Because Silver, inventor of time travel, only speaks English, and doesn't have time to learn a new language, so he hires Stewart, and a medical doctor would be pretty useful to bring along.
Why can't Silver know another language? Adding a random character just to fix a problem is just plain bad story telling; similar to a Deus ex machina. It's meaningless to magically give characters all the tools they need to survive the story as it takes away all suspense or makes a reader roll their eyes and think: "How convenient."
Leonid is 5'8'', faked his medical degree, and is wanted in Illinois for tampering with the mail. His original name is Nate. Stewart is 5'10'', taught ESL in Kenya for a year, and wants to find a way to time travel further back to ancient Rome to master his Latin.
I'm confused. You've already fleshed some other characters out, right? What's different about the process for these?
Surely a man who invented TIME travel has the TIME to learn a new language. That probably sounded sarcastic, but I'm actually wondering how the man who invents time travel does not have the time to do things. Surely, he'd have all the time in the world?
I often find that if I just write the story, the characters that are needed will show up. They can then be fleshed out through the interactions with the MC. If you want to flesh them out more, spend more time with them. Write some scenes for them that will not appear in your novel. That way you'll get to know them.