2D + Speed + Bonuses (reroll ties)
When the order of actions is uncertain, initiative must be rolled. Such a situation is when combat is beginning, and all parties are aware of each other. Initiative is determined by rolling 2 dice, and adding the sum to the character's speed and any modifiers that may exist.
Some characters may acquire an initiative bonus because of the occupation weapon specialist, otherwise the modifiers usually reflect on the accessibility of the weapon. If the weapon is in hand and in the open, then there is no detriment. Having the weapon hidden means a -1 on initiative, in a holster is an additional -1, in a belt or slung is -2, and -5 if in a bag or sack. Most referee's do not use the accessibility modifiers unless they relate to some kind of negotiation. Reaching for a weapon, or just removing the concealment can be considered threatening as well as making combat easier.
For dice pooling, it is best to only incur initiative checks at the very beginning or when there is an action-stopping interruption. For the rest of the combat, dice pooling only works when there is a continuum of action-reaction-action-reaction- and so on. This is because the dice pool is automatically reset when initiative is rolled. This will make much more sense if you read Dice Pooling.
If initiative is rolled for every turn, a great deal of realism can be maintained, but it can get confusing for a large number of participants. This can be smoothed by having the separate parties act in sequence, meaning that an entire party acts together.
An alternate method that is much quicker for large parties, though completely lacking in realism, is group initiative. Two dice are rolled, and added to the group's average speed. This is done for all distinct groups, and the totals indicate the order of actions group-wise.
The regular cycle of combat-time (action-defense-action-defense...etcetera) is sometimes best interrupted. For example, when an enemy is using their turns to move from one cover to another entirely in one turn, this leaves no opportunity to attack them while vulnerable between covers. If there is nothing compelling the attacker to check their defense (meaning, nobody is engaging with them), the attacker can forgo their action to instead wait for their enemy to try to do something. In the above example, when the enemy tries to run for the next cover there is forced an initiative check between the two to see who can act first - giving the attacker the possibility of interrupting the enemy's progression from cover to cover.
Inversely, if a combatant has a partner that can "provide cover" by continuing to engage an attacker every relevant turn, they prevent the attacker from being able to force initiative. It is up to the referee's discretion how much engagement is necessary to block a combatant from forcing initiative.