Tahini can appear to seize when lemon juice first touches it. The mixture becomes stiff, and adding more liquid feels like the wrong response. In many ordinary dressings, patient stirring and more water are exactly what is needed.
Start with well-stirred tahini. Add the lemon, then water a spoonful at a time, stirring thoroughly between additions. Watch for the paste to loosen into a smooth sauce. Stop at a thicker texture for a spread or a thinner one for drizzling.
Season after you reach the consistency you want. A dressing that tastes too strong when concentrated may be balanced once diluted. If it later thickens in the refrigerator, loosen the portion you need with a little water. Use clean utensils and appropriate chilled storage rather than repeatedly leaving the whole batch on the table.