Too many diffrent things can happen: Always let the Players Characters make the test.
- if its an obsidiman fleeing he will use his Str and momentum (try to stop 450kg running guy

) to push aside anyone blocking his way.
For complex mechanics I would: if a character is blocking his way, than compare Str of that character with obsidiman weight, use the Characteristics Table to find the lowest Strength Value that allows to Lift that weight (thats two times Carrying Capacity), and determine the difference to his own Strength Value. Add +6 to that number, and that would be Difficulty Number for Str Test of blocking character (Feats of Strength, PG p.249)
For much faster and simple mechanics: add +2 or -2 to Str Test Result for each weight category between characters: Obsidman pushing Troll (Obsidiman Str Test +2 vs Troll Str Step // Troll Str Test -2 vs Obsidiman Str Step). Obsidiman pushing Elf, Orc, Tskrang, Dwarf, Human - Str +4/-4. For Windling +6/-6, i would rather use +8/-8.
If attacked he will use defensive talents (Free Action), than use Delayed Action to delay his action into the following Combat Round (he does not act during the current Combat Round exept Free Action Talents). His Initiative for the following round is automatically 1 higher than the highest Initiative rolled for that round. And he keeps running
