What determines whether I can use a Paragon or Renegade Interrupt?

I made two characters and I noticed I get the Paragon Interrupt (the flashing icon that lets you interrupt a cutscene to say something Paragon/Renegade-y) when my character is mostly Paragon and I get Renegade when my character matches that. I haven't played with a neural character at all.

What determines which Interrupt you can perform? Do you just always get the Interrupt matching the morality you most commonly choose? Do I occasionally get the option of either Interrupt?

Solution 1:

From the ME wiki:

Unlike Charm and Intimidate, Shepard is not required to have any minimum amount of Paragon or Renegade points in order for an interrupt to be presented. All characters will encounter the interrupt at the appropriate time, assuming they made the correct dialogue choices to reach that interrupt as described below (if any).

Shepard always has the option to perform both kinds of interrupt. If you don't see a Paragon / Renegade interrupt, it's because there isn't one, not because of your character's reputation.

For instance, as a paragon Shepard, I was able to take the Renegade interrupts while boxing with James Vega.

What Mass Effect 3 has added is variable reputation interrupts, that change based on whether your morality leans blue or red. This does not mean, however, that you cannot perform renegade interrupts, or that you will never see renegade interrupts as a paragon Shepard - it just means that, on average, you will see more Paragon interrupts than Renegade.

Solution 2:

First off, Interrupts are scripted. Not every conversation has one, and not every opportunity to use a Paragon Interrupt has a corresponding Renegade Interrupt, and vice versa.

As for what determines whether you can use them, much like charm and intimidate options, there are occasionally minimum reputation or Paragon/Renegade scores that determine whether one is available to you. In other(/most?) cases, the interrupt will always be availiable, regardless of your existing reputation scores. Additionally, in some cases there are mirrored interrupt options, and the game will only provide you with the interrupt corresponding to the higher of your Paragon/Renegade scores.