By investigating the electrochemical nucleation and growth of polyaniline (PANI) on the insulating gap area of an interdigitated electrode, a template-free, in-situ approach is developed to obtain PANI nanowires and nanofibers with horizontal orientation on solid substrates. Experimental results show that the deposition process of PANI on the gap area is significantly influenced by polymerization conditions (such as polymerization current and time) and surface characteristics of the substrates (such as hydrophilicity/hydrophobicity and roughness). The concentration of solution-formed oligomers and the nucleation amount of the oligomers on the solid surface determine the morphology and orientation of the nanostructures. Controlling the deposition with high oligomer concentrations and limited nucleation amounts on the substrate is the key to the horizontal orientation. Gas sensing experiments confirm that the horizontal orientation of the nanostructures helps to improve the sensitivity and response time of sensor devices. Because of its simplicity, the approach proposed in this paper can be used in the fabrication of nanostructured conducting polymer chemiresistive gas sensor with high sensitivity but low cost.