I 2nd what
@bumbershoot said. DL meet & greets that are in a fixed location do have a definite line. DL characters that are roaming around? No...there's no formal line.
BUT...I would argue that if you change your perspective a bit and get "in the moment," you can turn those random "roaming character" moments into something pretty great! Maybe you won't get a photo op and an autograph out of it, but the memories you'll get from it are equally as valuable.
Here's an example...I couldn't find this one on Fresh Baked's
Youtube channel, but this clip gives credit to Fresh Baked.
I came upon the Evil Queen once about a year ago at DL. She was clearly walking with purpose to some sort of destination and didn't have time to stop and say hello. So I bowed down as she approached and said something like, "Good day, my queen." the Evil Queen didn't miss a beat and said, "Now THAT is a proper greeting!" and she continued on her way.
IT WAS AWESOME! AND SO MUCH FUN!
During our last vacation to DL, we were walking through Tomorrowland and, as you might be aware, there are often stormtroopers patrolling there in the afternoon. They don't have a standard Meet & Greet location. They roam and they interact with guests, but it's not like it's a "stop and take a photo and get an autograph" sort of thing. Anyway, I spotted the stormtroopers, and told my kids, "Follow me!" We caught up to them and I told them, "I have some rebel spies here that you should inspect." And the stormtroopers inspected my kids, called them rebel scum and cool stuff like that. It was a highlight of our trip. I snapped some quick pics while the stormtroopers were inspecting the kids, but it was not a staged thing or anything like that where they posed. It was just in the moment. And it didn't last for more than a minute or 2.
But you know what? My 9 yr old loved it. It was one of her favorites of the whole trip. And it was totally spontaneous. We didn't have to reserve a FP+ for it. It just happened.
LOTS of characters at DL "just roam." For example, we've seen Chip & Dale in Town Square. They will hang around and play with you if you engage them in more than a "can I get your autograph" sort of thing. Talk to them. Even if your kid(s) suddenly clam up and won't say a word. Just talk to the character and respond to their gestures as if you totally understand them. yes, you might feel like a fool sometimes doing it, but you're doing this for your and your family's enjoyment and trust me....your kids will LOVE IT!