After you assign it to the defending player, after assigning lethal damage to the creatures blocking the attacking creature with trample. Lethal damage only considers toughness, it ignores any abilities that would modify the actual damage dealt to the blocking creatures. (i.e. Double damage effects like Furnace of Rath, Damage prevention effects like protection from [quality], etc.)
First, it might be important to understand what trample does. Trample modifies the rules for how combat damage is assigned. Specifically, it allows trampling creatures to ignore the rule that they can only assign combat damage to the creatures blocking it.
702.18a Trample is a static ability that modifies the rules for assigning an attacking creature’s combat damage. ... (See rule 510, “Combat Damage Step.”)
Combat Damage Step
510.1c A blocked creature assigns its combat damage to the creatures blocking it. If no creatures are currently blocking it (if, for example, they were destroyed or removed from combat), it assigns no combat damage. ...
Trample and the Combat Damage Step, both describe how to calculate lethal damage.
[510.1c | 702.18b] ... When checking for assigned lethal damage, take into account damage already marked on the creature and damage from other creatures that’s being assigned during the same combat damage step, but not any abilities or effects that might change the amount of damage that’s actually dealt.
Lethal Damage - An amount of damage greater than or equal to a creature’s toughness. See rules 119.6, 510.1, and 704.5g.
Deathtouch is the only ability that modifies what lethal damage is.
702.2b Any nonzero amount of combat damage assigned to a creature by a source with deathtouch is considered to be lethal damage, regardless of that creature’s toughness. See rules 510.1c–d.
Finally, to answer your question, Under what circumstances does an attacking creature with trample assign damage to the defending player?
Your attacking creature is attacking a player, not a planeswalker.
Your attacking creature has additional combat damage that it can assign to the defending player after assigning lethal damage to all creatures blocking it.
You choose to actually assign the additional damage to the defending player. (Trample allows this, but it is not mandator to assign additional damage to the defending player instead of the creatures blocking it)
The defending player is still in the game. (important for multiplayer, where it might be advantageous to leave the game than allow an opponent's Lifelink creatures to damage you)
800.4e If combat damage would be assigned to a player who has left the game, that damage isn’t assigned.