Does cascade allow you to cast the exiled spell for free including without paying alternative costs?

Cascade (CR 702.84) instructs you to exile cards from the top of your library until you exile a nonland card with lesser mana value than the spell with cascade, then you may cast that card without paying its mana cost. 'Without paying its mana cost' is a specific permission granted by the rule, not a blanket waiver of all costs.

Crucially, CR 702.84c and the general rules on casting without paying mana costs (CR 117.9d) clarify that when you cast a spell without paying its mana cost, you may not pay alternative costs unless another rule or effect specifically allows it. So you cannot choose to pay a Flashback or Emerge cost instead — the 'free' cast is specifically 'without paying its mana cost,' and swapping to an alternative cost would require separate permission.

However, you still must pay any additional costs (e.g., targets that require sacrificing a permanent, or kicker if you choose to pay it). The cascade effect only waives the mana cost itself.

Concrete example: You cascade into Kavu Primarch, which has a kicker cost. You cast it without paying its {3}{G} mana cost, but you may still choose to pay the {3} kicker on top of casting it for free, because kicker is an optional additional cost, not an alternative cost.

In summary: cascade makes the exiled spell free in the sense of waiving its mana cost, but you cannot substitute an alternative cost, and optional additional costs like kicker can still be paid on top.