Rereading Citizen of the Galaxy for the umpteenth time since 1969 - luckily, my junior high school library carried it; there was no paperback edition in those days - I am reminded once again of how Heinlein's use of present tense at appropriate moments in the narration makes the story uniquely vivid, as though these other worlds exist now. It's a simple technique but works astonishingly well (for me at least).

This begins on the first page - "The slave market lies on the spaceport side of the famous Plaza of Liberty..." - and is very judiciously used all the way through the Great Gathering of the People, nearly two-thirds of the way through the book. The last narration that's purely present tense is just after a section break: four sentences beginning "A Great Gathering, although a time of fun and renewed friendships, has its serious purposes."

After that, it doesn't seem to be used at all, except once after Thorby is back on Earth, and that's a single verb within a sentence otherwise given to ordinary past-tense narration: "When they passed the immense pile which is headquarters of the Hegemonic Guard, Thorby had the driver slow down while he rubbernecked."

Becuse we readers are on Earth right now, present tense wouldn't work to "put us there" (i.e., on future Earth) the way it does for Jubbul, Sisu, and the planets Sisu visits and the habits of their natives (such as the Losians' method of urban transportation). But to Thorby, all of those places he's been are still "present-tense" places, just as we readers have come to regard them; now he has to put together all he's learned and decide how to act.

I can't recall offhand whether any of Heinlein's other non-first-person books use this technique, but it's damn effective in Citizen.