As a DM, your goal should be that everyone has fun. This includes being a referee if someone at your table is impeding on someone else's fun.
As others have said, not everyone is a roleplayer. That's fine, if you are putting out the effort for your 'watchers' to pick up on it, take some comfort in that. It's like fishing, try different baits(read: other angles), and you might catch one or more of them in it. You will remember that time and approach when it presents itslef in your game, because you will have fun, and so will your player, because they will have gone from watching the story to being the story. Don't expect full shakespearean performances (You don't go out expecting to catch a world record fish on the first cast, do you?) But be mindful of their involvement, and watch for signs of piqued interest, then pursue that to bring them in. This won't alway work, but is definitely worth a try.
On the flip side, talk to your 'Watchers' about their heckling. Explain that you are all there to have fun, and there is no reason that everyone should not be able to do it in their own way. Ask them to scale it back, because it is hurting your and other people's enjoyment. If they feel the need to do it suggest that they do it in character, that way they at least have to think and put some effort into it, and they may decide to forgo it if its too much effort, or you will get some roleplaying from them and your 'actors' can interact with them.
One thing to keep in mind is to speak privately with the people you see as disruptive, and politely explain your situation. If they respect you and your game, they should be able to accommodate your requests.
That's my two cents. Sorry for the all the fishing analogies, I don't know why they kept coming up.
As others have said, not everyone is a roleplayer. That's fine, if you are putting out the effort for your 'watchers' to pick up on it, take some comfort in that. It's like fishing, try different baits(read: other angles), and you might catch one or more of them in it. You will remember that time and approach when it presents itslef in your game, because you will have fun, and so will your player, because they will have gone from watching the story to being the story. Don't expect full shakespearean performances (You don't go out expecting to catch a world record fish on the first cast, do you?) But be mindful of their involvement, and watch for signs of piqued interest, then pursue that to bring them in. This won't alway work, but is definitely worth a try.
On the flip side, talk to your 'Watchers' about their heckling. Explain that you are all there to have fun, and there is no reason that everyone should not be able to do it in their own way. Ask them to scale it back, because it is hurting your and other people's enjoyment. If they feel the need to do it suggest that they do it in character, that way they at least have to think and put some effort into it, and they may decide to forgo it if its too much effort, or you will get some roleplaying from them and your 'actors' can interact with them.
One thing to keep in mind is to speak privately with the people you see as disruptive, and politely explain your situation. If they respect you and your game, they should be able to accommodate your requests.
That's my two cents. Sorry for the all the fishing analogies, I don't know why they kept coming up.