What was the guard’s initial reaction to the arrival of Jerry Cruncher?

In Book I, Ch. 2 of Dickens' "A Tale of Two Cities,"  entitled "The Mail" Mr. Jarvis Lorry a confidential clerk of Tellson's Bank is on his way to Dover in the horse driven coach "The Dover Mail." It is around elven on a cold November night and a fog envelops the coach as it struggles to climb up Shooter's Hill. Visibility is practically zero:

"There was a steaming mist in all the hollows, and it had roamed in its forlornness up the hill, like an evil spirit, seeking rest and finding none. A clammy and intensely cold mist, made its slow way through the air in ripples that visibly followed and overspread one another, as the waves of an unwholesome sea might do. It was dense enough to shut out everything from the light of the coach-lamps but these its own workings and a few yards of road; and the reek of the labouring horse steamed into it, as if they had made it all."

The passengers had alighted and the coach man Tom and his guard Joe with great difficulty were struggling to push the coach uphill. Just then they heard the sound of a horse galloping towards them. They immediately suspected the rider to be a highway man. The guard of the "Dover Mail" immediately cocked the safety of his blunderbuss and aimed it in the dark in the direction of the horse rider and threatened to shoot him. However, Jerry Cruncher identifies himself as a messenger from Tellson's Bank, London with a message for one Mr. Jarvis Lorry. Mr. Lorry assures the coachman and the guard that there is nothing to be frightened of and receives the message from Jerry Cruncher and reads  it and replies to it. Jerry then returns to London and the Dover Mail continues its journey towards Dover.

This incident reveals to us how unsafe the roads in England were during Dickens' time and how each passenger feared for his own safety:

In those days, travellers were very shy of being confidential on short notice, for anybody on the road might be a robber or in league with robbers.