If you take an action that qualifies as either of your additional actions, e.g. from Leo De Luca and Quick Thinking, you may choose which additional action you are actually using.
- On the 'additional action': When spending actions during your turn, the first action you spend each turn takes up the 'additional' designation. If Leo De Luca is discarded mid-turn, you do not lose 1 action from your total pool of actions for the turn. If you then play a second copy of the card, you gain a further additional action.
-- example: "Skids" O'Toole has Leo De Luca in play and is engaged with a Ghoul Minion. First action, Skids attacks the ghoul (this is the additional action from Leo). Second action, Skids moves location, provoking an attack of opportunity from the ghoul. The investigator places the damage on Leo from the ghoul's attack. Third action, Skids moves again, provoking another attack of opportunity from the ghoul, which kills Leo. Fourth action, Skids plays a second copy of Leo, provoking an attack of opportunity from the ghoul, which the investigator takes on Skids. Fifth action (the 'second' additional action from the second Leo), Skids attacks the ghoul and kills it. [No idea why you'd do it this way but hey it's an example.]