Many of us have heard the old saying ..... "It's not just what you say, but how you say it that counts"
Both the words that we use, and how we deliver them, will have an influence on whether other people take us seriously.
Whether we are expressing an idea at work to our boss, or voicing a preference to our partner about what movie to go and see, or even making a complaint as a consumer about poor service - what-ever the situation, there are some common ingredients that need to be present if our communication is going to be assertive in nature.
What are these ingredients for assertive communication? Well for a start, looking at the other person with reasonable eye contact, combined with a calm steady voice, is a good foundation. And we need to be speaking loudly enough to be clearly heard .... just like in Goldilocks, "not too loud, and not too soft - but just right".
If you speak too loud, you may come across as aggressive. Speak too softly and you may appear to lack confidence.
Same with eye contact - if it's too intense, you may come across as aggressive, and if you don't look them in the eye, you may seem to lack conviction. Diffferent cultures do have different values around eye contact, but generally in our western culture reasonable eye contact is needed for assertive communication.