Rules

Actions

Helping and Skill Checks

In general the player that prompts the check makes the roll.

See Skill check and Dogpiling

Improvise Action

When a character tries something interesting during combat it will cost either an action or bonus action but only if it is successful. If the check fails, it costs nothing. In general there is no penalty for failure though risky actions may incur things like falling prone or taking falling damage.

Mechanics

Death Saves

Death Saves are made by the player behind the DM screen. Only the player and the DM know the results.

Spellcasting

less pedantic
If you have a free a free hand and component pouch or are holding a spell focus you can cast any spell with material or somatic components. Otherwise you can only cast spells with verbal components.