From Project: Redcap
Terminology: Initiative Score and Initiative Total
To be precise, a character's Initiative Score is a fixed number written on the character sheet. In combat, a player rolls a Stress Die and adds the character's Initiative Score. The result is called an Initiative Total.
When to Roll Initiative
In Fifth Edition, each character rolls Initiative only once, at the start of Combat (or Certamen). Initiative may also be used at the start any action scene where it matters which character acts first: for example, a chase scene. The character with the highest Initiative Total acts first, and then other characters act in descending order of their Initiative Total.
On subsequent rounds, do not roll Initiative again. All characters simply get another turn, following the same sequence, and keep acting in the same order until the end of the battle. This works pretty much the same as Third Edition D&D, for players who are familiar with that game.
Initiative in Earlier Editions
In Fourth Edition, Initiative was rolled during the Movement phase of the combat round (page 164). If two characters wanted to engage each other, there would be an opposed roll and the character with the higher Initiative Total would determine the range at which the characters engaged. Furthermore, the winner of the engagement contest got a bonus to his Attack score. In practice this meant Initiative was rolled frequently, but not necessarily every round.
If two or more characters have the same Initiative Total, roll again to break the ties. This tie-breaking roll only affects the relative order of the characters whose Initiative Totals were tied: it does not supersede the original Initiative roll (see Example of Initiative, ArM5 page 172).
[anchor:Spells]Initiative and Spellcasting
For regular formulaic or spontaneous spellcasting (as opposed to Fast Casting), a magus casts spells at his normal turn in the round as determined by his initiative roll. On page 83, under "Magic" (as a combat option), the rules say spells have no weapon initiative modifier so spellcasting initiative is a stress die plus Quickness. The spell takes effect immediately once you cast it.
[anchor:ChangeWeapons]Initiative and Changing Weapons
There are no official rules for this - it's probably another thing that was left out for simplicity or brevity. Common sense suggests that a character who changes weapons shouldn't get a free re-roll of Initiative. Instead, just change the character's Initiative count by the difference between the Init bonuses of his old weapon and his new weapon.
For example, | Gerard the Turb Warrior, the famous Fifth Edition sample character, confronts a pair of highwaymen. He wants to dissuade them from robbing him, but not necessarily cripple them, so he uses his staff instead of his mace. He rolls a 5 for Initiative and his staff's Init bonus is +1, for a total of 6. He fights for a couple of rounds until two more highwaymen spring from the bushes. Now seriously threatened, he drops the staff and whips out his mace. We'll pretend he forgot what his actual die roll was but knows that he goes on a 6 because the storyguide wrote that down. The mace's Init bonus is one lower than the staff's, so Gerard goes on initiative 5 for as long as he uses his mace.
The history of this page before August 6, 2010 is archived at Legacy:initiative