There are addons that can change the look of the casting bar.
Cast time (or casting time) is the time needed to cast a spell or ability before it takes effect.
Spells and abilities can be categorized as:
- Instant: These abilities occur instantly and do not have a cast time. They therefore cannot be interrupted and so are harder to counteract in PvP situations.
- Cast-time: Most spells are of this type. When casting cast-time spells or abilities, a time bar will appear and fill up (with yellow) during the casting time, and the spell effect will not happen until the bar fills completely (at which point it turns green). If the cast is interrupted, it will fail and no effect will occur. Pushback from damage taken can also slow the cast, increasing the time it takes for the spell to occur. Cast time spells are slower to occur and easier to counteract in PvP, since opponents can see your casts coming.
- Channeled: Channeled spells do not have a cast time, and begin immediately. However, their effect is measured out in several "ticks" over the time of the channel, with the first tick usually not occurring for at least a second after the channel begins. Damage taken while channeling will foreshorten the spell, but will not delay its effect like with cast-time spells. Almost all channeled spells are treated as periodic effects (damage over time or heal over time).
The casting time of an ability is reduced by the player's casting speed (spell haste). For example, a player with 10% spell haste will find that their normally 1.5-second cast spells (such as 
[Flash Heal] or 
[Scorch]) have a cast time of only 1.36 seconds. This change is reflected in abilities' tooltips. Spell haste may be permanent or temporary and can be acquired from a number of sources.
Notes
- The player can interrupt their own spell at any time during the casting by either moving, jumping, or pressing the Escape key.
- Some non-spell abilities such as
[Slam] and 
[Throw] are also cast-time abilities. - No time bar will appear for instant spells.
- Melee attacks and direct damage taken by the caster will set back their spell's cast time by a given amount, slowing the casting. This is known as pushback.
- Various abilities can be used to interrupt casting completely. If this happens the time bar will fill completely with red and say "Interrupted".
- Mana is not consumed until the cast is completed. For instant and channeled spells this means that the full mana-cost of the ability is immediately consumed upon casting. For cast-time spells, mana will not be consumed at all until the cast is completed.
- Certain abilities, powers, spells, and talents can reduce casting time of specific spells (
[Serendipity]), make them instant cast (
[Nature's Swiftness]) or prevent interruption (
[Icy Veins]). - Any effect that increases spell haste will reduce the cast time of non-instant abilities, as well as reducing the global cooldown.