D&D5 - The bad guy should not be surprised.
PHB, p.189 - Surprise
A band of adventurers sneaks up on a bandit camp, springing from the trees to attack them. A gelatinous cube glides down a dungeon passage, unnoticed by the adventurers until the cube engulfs one of them. In these situations, one side of the battle gains surprise over the other.
The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.
As @KorvinStarmast stated, there is no such thing as a surprise round in 5E. A surprised character is simply not able to move, take an action or a reaction on his first turn of the combat.
In your situation, the bad guy is clearly facing the characters when doing his speech, so unless he is fanatically speaking to his hidden god of evil somewhere in the skies, he will see the character doing his move and won't be surprised.
If the party was speaking with a friendly noble NPC, for example, and a PC decided to attack suddenly, the NPC should be surprised, because he wasn't aware of the threat. When a BBEG faces a party of characters, it seems obvious that they are antagonists/foes/enemies and that each side is a threat to the other. In the end, it is the definition of threat that makes a difference.
Also, giving the player a round during which he alone can act might lead others to abuse this mechanic.
This is what the PHB says about scenarios that demonstrate one group surprising another:
A band of adventurers sneaks up on a bandit camp, springing from the trees to attack them. A gelatinous cube glides down a dungeon passage, unnoticed by the adventurers until the cube engulfs one of them. In these situations, one side of the battle gains surprise over the other.
So it's being shown to us that the encounter wherein creatures can be considered surprised begins when one group takes hostile action against the other group that has not noticed the first. And note that surprise can only happen when you are in the first round of the encounter, which means that, if you want to take advantage of surprise, Initiative cannot be rolled before the start of the ambush.
It also doesn't make sense to give someone the surprised status when nothing is surprising them (ie, the players are planning, not ambushing). When you are surprised, you cannot move. Does the act of the players planning somehow paralyze the other group? No, it shouldn't.
Best Answer
There is still certainly "surprise", though not quite a "surprise round".
See the surprise rules.
It is generally up to the DM whether there is a chance that one side (or some of the combatants) may be surprised.
The rules given are keyed towards one side being stealthy (rather than disguised) and so it becomes a Dex(Stealth) check vs a Wis(Perception) check. Initiatives are still rolled for everyone, but anyone surprised is unable to take actions on their first turn of combat.
For your disguise tactic, a DM might shake things up and ask for the party members to make Cha(Deception) checks vs a Wis(Perception or Insight) check of the enemy in order to determine surprise instead. But this would be up to the DM and veers away slightly from the surprise rules exactly as written.