Some actions such as readying a weapon, deactivating the tractor, expending a battery, or spending your action to move require no skill check. These are automatic actions. You must spend your action to take them but needn’t roll the dice.
Some actions are so simple and easy that they don’t even require you to spend your action to do them. Unless otherwise stated, a Free action must be taken on your turn. You can take it before or after your move or standard action or another free action you might get.
You may not take free actions during movement unless specifically allowed by the action or an ability. You may spend a standard action to go on overwatch to do a free action such as dropping something or letting go of the ship to become a spacewalker.
Example of free actions include:
Some special abilities offer free actions with certain restrictions. Unless specified within the description, the free action is restricted as described here.
Any number of characters that exit the same airlock on the same phase may be considered “linked up” for purposes of being tractored or picked up in a rescue pod. Those characters are all adjacent to one another until they declare otherwise.
Sometimes a character will be forced to make a skill check to resist something such as being disintegrated, stunned, or suffering unsupported Life Support damage if spacewalking without an EVA. These do not require an action and are not subject to penalties such as OOC. Note that drugs affect passive skill checks.
These actions have some similarities but marked differences.
Preparing costs you your action while Overwatch suspends your action. If you prepare to shoot on phase one, you get to shoot (at -1 difficulty) on phase 2 during character actions as your action.
If you go on overwatch to shoot on phase one, you may shoot at any time up until character actions on phase 2 and then take an action on phase 2 (which could be shooting again!)