How Interactive Is Modern

Interactivity is the extent to which a deck can respond to plays that the opponent makes. In all formats this is determined by the presence and power level of removal spells, discard spells, counterspells, tempo spells and the prominence of creature combat. Not having spells that enable you to interact with your opponent can mean a certain loss against combo decks that have the capability to win the game in a single turn.
In Modern, the removal spells are nearly on par with those of Legacy, missing only Swords to Plowshares and Toxic Deluge. The best removal is cheap, efficient and can kill a wide range of creatures, or even other permanent types such as artifacts
Counterspells are also a large part of Modern decks. Combo, control, and even some aggressive decks use them to protect their combo pieces, protect themselves, or protect their creatures.
Tempo spells are those that temporarily slow down the opponent, typically by returning something to hand in order to force the opponent to recast it (using their mana inefficiently). This wasted time is used by the tempo player to cast or attack with aggressive threats such as Young Pyromancer or Delver of Secrets , taking advantage of this momentary lead in board presence. The most representative example of this is Remand, which has already been discussed.
Most black decks have access to discard spells that allow them to heavily disrupt combo and control decks. Some discard spells allow you to look at the opponent’s hand and choose what they discard. The advantage of these cards is that, unlike killspells and counterspells, they give you information about what the opponent has in hand and what they are likely to cast in the future.
Creature combat is a very important part of Modern. There are several prominent aggressive decks, including Affinity, UR Delver, Burn/RDW and Naya Zoo which focus on cheap, efficient creatures such as Wild Nacatl, Goblin Guide, and Delver of Secrets , or strong synergy to pressure the opponent into blocking or using removal spells. Most aggressive decks can win by turn 4 if left unchecked.