First of all, musketeers have fire at will option. If you enable it you will get a more or less continuous fire.

Second, if you have them on fire at will they wont fire into friendly troops. Which means that if you have them on level ground behind your engaged troops they wont fire at all.

If you have them on a hill behind your lines they will fire, but ofc they still can get friendly kills (though not much).

Best place is a cliff overlooking the battlefield.