Natural 20

Natural 20s in combat are considered a Crit and an automatic hit.
In combat the crit equals 2 x base damage + 1 x maxed dice + 1 x rolled dice
(e.g. you crit and your weapon normally deals 1d6 + 2, your crit roll = 6 (max roll of 1d6)+ 1d6 + 4 (2x2))

Out of combat, role play natural 20s can result in Rule of Cool opportunities, however in a contested roll they are not an automatic success.
Investigation/perception/insight checks at 20 are also not considered 100% crit successes.
Where possible I'll play with a level of transparency.