It’s very important to set a tone on the first impression, but the tone must also be maintained.
When you first meet someone it’s best to be on the best behavior because this is how credibility is built. “My parents taught me to be on my best behavior at all times”. In life people make their judgment about you based on what they see or what someone else think.
It’s a big task to maintain credibility and an easy task to lose credibility. I had a boss who seemed to be very structured but when it was time to make commitments he would never follow through. He also had a problem with delivering information, when writing emails with error, he would point the finger. Pointing the finger causes a negative reaction, this had caused the entire organization to lose trust and his credibility had turned weak.
I think being trustworthy and honest plays a huge role. If you carry a professional demeanor all the time, you have a higher chance of building and maintaining credibility.