This topic is not designed to tell you exactly how to role play your Scarlet Crusader.
Most of us know that The Scarlet Crusade has a tragical and very humane background behind their wickedness and bloodthurst, and yet they try to fulfill a noble cause even if they will have to purge a whole nation with sword and fire.
Remember that every one who joined the Scarlet Crusade by his own will, had some personal reason for it, will it be grief for the fallen homeland, death of the family, beloved or general hatred to the Undead. Some also may join because they belive that Scarlet Crusade alone is doing the right thing or they are dedicated to serve the Light.
Stay Humble. The Crusade consists of many poor equipped zealots, who are basically enraged farmers and citizens. Their training is also leaves much to be disired. Most of them are being sent in a field as spare manpower and dont hold for too long. Yet those who perciever, recieve recognition and better training, emerging as scarlet crusaders. Understand, that You cannot join the Crusade and be trusted or recognised enough to wear outstanding armor. Represent the vision and quality of the Crusade as it is. The key to interesting Scarlet Crusade role play is not to look immortal champion of Light, but the contrary, be believable.
Speak less & Act more. You don't get Scarlet Crusaders to talk to you a load of reasoning before they launch an attack. Their vision is obviously clouded by hatred and they see all in "Black and White" (meaning white is scarlet and black is every one else) If the approaching stranger is armed, looking like possesses threat or non-human, attack him or her on sight, only supplying it with reason that what is before is only undead. (in rarest of cases, leave talking to an officer)
Faith and Spirituality: The Scarlet Crusade still believes it embodies Light's will and what they are doing is righteous, therefore the Light has not abandoned them. There are people of many crafts who fulfill their role in the Crusade, some may not be strong believers, or not at all, but Light is a major engine which moves the Machine of the Crusade and keeps it in one piece. Openly disrespecting the Light may get other crusaders become aggrivated at your character or may lead to even more bitter results. It serves well to sometimes mention the Light and join the worshiping circles organized by Clergy.
Behaviour. This is generally up to you, but as you can mostly learn and see from the Scarlet NPC's it can variate from untamed ferocity of Herod, self-righteousness of Abbendis to sadistic cruelty of Issilen. There is ofcourse big array of personality types in the Crusade, and in same way, such can find their place in our guild. In general scarlet crusaders are sacraficial and determinated, all depends on what you are.
While thinking on your character's behavior dont forget the zealousness backed by general feeling of insanity and paranoia dwelling inside the mind of most scarlets. Hatred to all non-human races is also present.