You can look at the top card of your library whenever you want (with one restriction; see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.

If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you can't look at the new top card until you finish doing so. This means that if you cast a spell from the top of your library, you can't look at the next one until you're done paying for that spell.

You must pay all costs and follow all timing rules for spells cast from the top of your library this way.

If Assemble the Players leaves the battlefield and returns to the battlefield in the same turn, the casting permission granted by the old one is different than the casting permission granted by the new one. Similarly, if you control multiple Assemble the Players, each one has a different casting permission that may be used once each turn.

Because you never "cast" a land card, Assemble the Players doesn't allow you to play a land creature (such as Dryad Arbor) from the top of your library.

If the top card of your library has a disguise or morph ability, you can cast it face down from the top of your library, even if it doesn't normally have power 2 or less.