Understanding the biological self-assembly process of virus capsids is key to understanding the viral life cycle, as well as serving as a platform for the design of assembly-based antiviral drugs. Here we identify and characterize the phenylpropenamide family of small molecules, known to have antiviral activity in vivo, as assembly effectors of the hepatitis B virus (HBV) capsid. We have found two representative phenylpropenamides to be assembly accelerators, increasing the rate of assembly with only modest increases in the stability of the HBV capsids; these data provide a physical-chemical basis for their antiviral activity. Unlike previously described HBV assembly effectors, the phenylpropenamides do not misdirect assembly; rather, the accelerated reactions proceed on-path to produce morphologically normal capsids. However, capsid assembly in the presence of phenylpropenamides is characterized by kinetic trapping of assembly intermediates. These traps resolve under conditions close to physiological, but we found that trapped intermediates persist under conditions that favor phenylpropenamide binding and strong core protein-protein interactions. The phenylpropenamides serve as chemical probes of the HBV capsid assembly pathway by trapping on-path assembly intermediates, illustrating the governing influence of reaction kinetics on capsid assembly.